The invention is based on a brushless, electrically commutated direct-current machine, in particular a direct-current motor.
In a brushless direct-current machine of this kind, which is described for example as a direct-current motor in DE 37 09 168 A1, the electronic power switches, which are embodied as transistors in each winding phase, are disposed on the high-voltage side of the winding phase, connected in series with it. The power switches are successively connected to the network direct-current by the control unit, which includes a control stage for generating switching signals and a logic stage for applying the switching signals to the power switches in the correct sequence.
When field effect transistors, in particular MOSFETs, are used as low-cost electronic power switches, they are disposed on the low-voltage side of the winding phases as so-called low-side switches.
In a direct-current machine of this kind, if a power switch fails, then two possible malfunctions can occur: the power switch is shut completely off, or the power switch is conductive and conducts constant current, with or without residual resistance. In the first instance, the function of the direct-current machine is limited or does not function at all, but this is not critical. The second instance can give rise to a dangerous situation as the stator winding of the machine and the power switches heat up and can cause a fire as a result of the constant current. The temperature-sensitive fuses sometimes provided in the supply line to the machine cannot solve this problem because they have to be designed for a current that can also be greater than this constant current. In addition, due to their series divergence, fuses conduct up to seven times their nominal current so that there is the danger that the stator winding and/or the electrical circuit will start to burn before the fuse reacts.
The brushless direct-current machine according to the Invention, has the advantage that due to the heating, the fuse wire melts much more rapidly than when it is only heated by the machine or motor current flowing through it. Another advantage is that during normal operation, due to the extremely low ohmic resistance of the fuse wire, the fuse does not represent an additional resistance in the electrical circuit that might negatively influence the efficiency of the machine or motor.
According to a preferred embodiment of the invention, the heating current for electrically heating the fuse is conveyed by means of the fuse wire itself. In addition to heating the fuse wire, this measure further increases the flow of current through the fuse wire by the amount of the heating current required for the heating so that the melting temperature is reached much more rapidly. This also produces the advantage that the heating current is switched off automatically when the fuse wire melts.
According to an advantageous embodiment of the invention, in order to electrically heat the fuse, a heating wire or a heating cartridge extends along the fuse wire and is connected in series with an electronic switch, which closes in the event of a malfunction, connected in parallel with the stator winding. The electronic switch here is advantageously controlled by the control unit for the electronic power switches in the stator winding.
According to an advantageous embodiment of the invention, the time interval from the occurrence of the malfunction to the reaction of the fuse is further reduced in that the control unit is designed so that in the event of a malfunction, it sends a control signal to all of the power switches that causes them to close. As a result, the current flowing through the fuse wire is increased to an extremely high level and the melting temperature is reached in an extremely short period of time.
According to an advantageous embodiment of the invention, a pyrotechnic percussion cap that can be ignited by being heated can alternatively or additionally be disposed on or near the fuse wire and is connected in a thermally conductive manner to the heating wire. The ignition of the percussion cap breaks the fuse wire and switches the machine off. The heating current required to make the percussion cap react can therefore be designed to be a great deal lower than the beating current required to heat the fuse wire of the fuse.
According to an advantageous embodiment of the invention, the power supply for the control unit is tapped between the fuse and the stator winding, preferably before a capacitor used for smoothing. This has the advantage that when the stator winding is disconnected from the direct-current network by the reacting fuse, the power supply to the control unit is also switched off.